Interpretive Summary: Dual-energy X-ray absorptiometry (DXA) is often used as a reference method for body composition measurements. We have previously shown that a new version of analysis software (version 12.1) by Hologic, the manufacturer of the DXA instruments used in our facility, will affect bone mineral results for smaller subjects – the new software uses body weight to correct scans of subjects weighing less than 40 kg. We used the new software to re-analyze over 1350 scans of both boys and girls, from 2 to 18 years old, previously analyzed by our original software (version 11.2). We wished to assess the impact of the new software on soft-tissue body composition estimates in children. We applied several statistical tests to the results, and compared the old and new results for fat, percentage fat, and lean body mass according to the children's ages and body weights. The new software produced values that were higher for body fat and lower for lean body mass in children weighing less than 40 kg. Body composition values for younger, smaller children were most affected, and girls were more affected than boys. Using the new software, 14% of the girls and 10% of the boys were reclassified from the "normal" percentage body fat range to the "at risk of obesity" category, while 7% and 5%, respectively, were reclassified as "obese". Hologic's newest DXA software has a significant effect on body fat and lean body mass results for children weighing less than 40 kg. The effect is greater for girls than for boys. Investigators should use caution when comparing newly acquired body fat estimates with previous studies that used older DXA instruments and software. DXA has not yet achieved sufficient reliability to be considered a "gold standard" for body composition assessment in pediatric studies.

Technical Abstract:
Dual-energy X-ray absorptiometry (DXA) is often cited as a criterion method for body composition measurements. We have previously shown that a new DXA software version (Hologic Discovery V12.1) will affect whole-body bone mineral results for subjects weighing less than 40 kg. We wished to re-analyze pediatric whole-body scans in order to assess the impact of the new software on pediatric soft-tissue body composition estimates. We re-analyzed 1384 pediatric scans (ages 1.7-17.2 y) using Hologic software V12.1, previously analyzed using V11.2. Regression analysis and ANCOVA were used to compare body fat (TBF), percentage fat (%BF), and non-bone lean body mass for the two versions, adjusting for gender, age, and weight. Software V12.1 yielded values that were higher for TBF, lower for LBM, and unchanged for DXA-derived weight in subjects weighing less than 40 kg. Body composition values for younger, smaller subjects were most affected, and girls were more affected than boys. Using the new software, 14% of the girls and 10% of the boys were reclassified from the "normal" %BF range to "at risk of obesity", while 7% and 5%, respectively, were reclassified as obese. Hologic's newest DXA software has a significant effect on soft-tissue results for children weighing less than 40 kg. The effect is greater for girls than boys. Comparison of TBF estimates with previous studies that use older DXA instruments and software should be done with caution. DXA has not yet achieved sufficient reliability to be considered a "gold standard" for body composition assessment in pediatric studies.